added support for the --mexflags option. needs still testing

cmake/PROJECT_NAME-config.in.in

 #!/bin/sh
 
 print_usage(){
-    echo "usage: $0 [--cppflags] [--ldflags] [--version]" 1>&2
+    echo -n "usage: $0 [--version] [--cppflags] [--ldflags]" 1>&2
+    if [ -n "@@PROJECT_NAME@_MEXFLAGS@" ]; then
+      echo " [--mexflags]" 1>&2
+      echo -n " The mexflags are only for use with MATLABs mex." 1>&2
+    fi
+    echo ""
 }
 
 if  [ $# -eq 0 ]; then
@@ -23,6 +28,9 @@ while [ $# -gt 0 ]; do
       --version)
         OUTPUT="${OUTPUT}@@PROJECT_NAME@_SOVERSION@"
         shift;;
+      --mexflags)
+        OUTPUT="${OUTPUT}@@PROJECT_NAME@_MEXFLAGS@"
+        shift;;
       *)
         echo "invalid option: $1" 1>&2
         print_usage